CHANGES IN TREMATODE INFESTATION RATES ACROSS PARASEQUENCES: INSIGHTS FROM HOLOCENE DEPOSITS OF THE PO COASTAL PLAIN, ITALY

Previous work in Late Pleistocene–Holocene sequences from the Po plain revealed significant long-term fluctuations in trematode prevalence in marine environments: higher prevalence in retrogradational environments (TST) and negligible prevalence in progradational environments (HST). Here we expand upon this work by investigating traces of trematode parasitism among bivalves within parasequence sets from the Holocene brackish deposits of the Po Plain. The investigated portion of core 204-S7 is 24 m long and, from bottom to top, includes 5 m of amalgamated fluvial channel sands (latest Pleistocene) followed upwards by alternating packages of swamp and poorly drained floodplain deposits. The middle part of the core includes brackish deposits (inner to outer lagoon) accumulated through the Holocene. Using dense sampling (32 bulk samples and >1,550 specimens) of wetland and lagoonal strata from core 204-S7, we documented trematode occurrences in Abra segmentum, Loripes orbiculatus, and Cerastoderma glaucum. A randomization (10,000 iterations) at the level of individual specimens was conducted to investigate the statistical significance of the observed variability in trematode frequency. Frequencies of trematode infestation of A. segmentum were significantly elevated (p < 0.01) in samples associated with flooding surfaces and significantly depressed (p < 0.01) in samples collected from in-between flooding surfaces. Frequencies of termatode occurrences were similarly elevated around flooding surfaces in the case of L. orbiculatus. However, frequencies of trematode infestations in C. glaucum did not vary signinficantly across samples likely refelcting their low abundance. These results, documented at a much finer scale of observation within the sequence stratigraphic framework (dissecting parasequences) than in our previous studies (comparing data pooled by systems tracts), further support the hypothesis that increasing trematode prevalence is linked to transgressive cycles, a pattern now documented in shallow marine and estuarine settings, on two continents, and in both modern and fossil taxa.